Semiconductor translating devices



Feb. 17, 1959 B. SCHWARTZ 2,874,076

' SEMICONDUCTOR TRANSLATING DEVICES Original Filed Aug. 18, 1955 Bertram Schwartz,

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United States Patent SEMICONDUCTOR TRANSLATING DEVICES Bertram Schwartz, Los Angeles,-Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Original application August 18, 1955, Serial No. 529,351,

now Patent No. 2,832,702, dated April 29, 1958. Divided and this application November 29, 1957, Serial No. 699,830

Claims. (Cl. 117-200) This invention relates to fabrication of semiconductor signal translating devices and, more particularly, to surface treated semiconductor crystal bodies for use in such devices.

This application is a divisional application of copending United States patent application, Serial Number 529,351, f entitled Method of Treating Semiconductor Bodies for.

p 2,874,076 .Patented Feb. 17, 1959 opposite conductivity type separated by a P-N junction. A coating is provided on at least one surface of the body which consists essentially of a polymerized organo substituted silane.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description taken in conjunction with the accompanying drawing in which there is illustrated one embodi- V ment of the invention.

Referring to the drawing, there is illustrated in the single figure a typical semiconductor device comprising a semiconductor body 11 of one conductivity typehaving a region 14 therein of the opposite conductivity type. Body 11 and region 14 are separated by a P-N junction 13. An alloy region 15 is ohmically atlixed to region 14. The device as illustrated may be manufactured in accordance with United States Patent No. 2,789,068, issued to J.

Maserjian April 16, 1957. Upon one surface of the illustrated semiconductor device is an organo-substituted Translating Devices by Bertram Schwartz, filcd-August 18, 1955, now Patent No. 2,832,702.

In the semiconductor art, a region of semiconductor material containing an excess of donor impurities and having an excess of free electrons is considered to be an N- type region, while a P-type region is one containing an excess of acceptor impurities resulting in a deficit of electrons, or stated differently, an excess of holes. When a continuous solid specimen of semiconductor material has an N-type region adjacent a P-type region, the boundaryv well known to the art.

between the two regions is termed a P-N (or N-P) junction and the specimen of semiconductor material is termed a P-N junction semiconductor device. Such a P-N junction device may be used as a rectifier. A specimen having two N-type regions separated by a P-type region, for example, is termed an N-P-N junction semiconductor device or transistor, while a specimen having two P-type regions separated by an N-type region is termed a P-N-P junction semiconductor device or transistor.

As is now well known to the art, germanium and silicon crystal bodies are used in semiconductor translating devices, such as rectifiers, transistors and photocells. It is also well known to the art that the semiconductor devices are adversely affected by the presence of moisture on the surface of the semiconductor crystal body.

impervious to moisture which may precipitate or deposit upon exposure of the semiconductor body to ambient conditions, or by other causes. The most common prior method of rendering the surface of the semiconductor body moisture resistant is by coating flie surface of the" body with silicone varnish and baking the body at an elevated temperature for an extended period of time. For example, the common baking procedure after application of the silicone varnish is to bake the crystal body for a period of four hours at a temperature of 300 C. This method has several limitations and disadvantages in pro- Various means have been utilized, therefore, to render the crystal" silane coating 12. The coating and the processes for applying it will be described more fully hereinafter. It should be expressly understood that the device as shown and the location of the coating thereon is illustrative only and is not to be taken as a limitation of the scope of the present invention.

In accordance with an illustrative embodiment of the present invention, a silicon crystal 11 which has been cut and lapped to the desired dimensions is etched by methods For example, in the presently preferred embodiment, the etching step is carried out by immersing the silicon body for approximately 30 seconds in a solution containing equal parts of nitric acid, hydrochloric acid and acetic acid. The silicon body is then rinsed in distilled water, boiled in a 50/50 mixture of acetone and methyl alcohol and rinsed in absolute methyl alcohol.

After etching as described above, the surfaces of the crystal body are moistened by immersing the body in water and then drying with filter paper to remove excess water. Since the reaction between water and organo substituted silanes causes polymerization of the silane, more uniform results are obtained in surface treatment when moisture is present on the surface. Visible condensed water, however, should be removed since its presence causes the formation of thick non-uniform patches of the surface coating which is applied as described hereinafter. A water hydrolyzable organo-substituted silane is then applied to all surfaces of the silicon body. In this embodiment this is accomplished by immersing the silicon v body in the organo-substituted silane liquid. The silicon body is left in the liquid for a sufficient length of time to cause complete wetting of all surfaces. For example,

one minute is an illustrative length of time. For the most uniform results the liquid silane is agitated to cause complete wetting of the silicon surfaces by the silane. Excellent results have been achieved by using ultrasonic agitation. In the presently preferred embodiment a mixture of organo-substituted chlorosilane is used which is a mixture containing equal parts of methyltrichlorosilane and dimethyldichlorosilane. The use of this mixture as the water hydrolyzable organo chloro-substituted silane liquid furnishes space polymerization and maximum bonding of silicone molecules to the silicon surface. The reaction of the dimethyldichlorosilane and methyltrichlorosilane with moisture which is adsorbed on the surface of the silicon body causes hydrochloric acid to be splitoif of one conductivity type having therein a region of the and leaves a thin water repellent film 12 of silicone polymer which adheres to the silicon surface. The organic groups present in the silicone polymer, whichare methyl groups in the presently preferred embodiment,

furnish a hydrophobic surface which resists Wetting by moisture. The mixture of dimethyldichlorosilane and methyltrichlorosilane results in excellent surface treatment of the silicon crystal since the methyltrichlorosilane furnishes a spatial 3-dimensional polymerization chain for good bonding and complete coverage of the silicon surface, while the dimethyldichlorosilane supplies a maximum number of organic methyl groups for good moisture repellency. In addition, the chemical bond between silicon atoms and carbon atoms in the silicone polymer is strong, resulting in good thermal stability of the film.

It will be apparent to one skilled in the art that although a 50/50 mixture of dimethyldichlorosilane and methyltrichlorosilane has been described in the illustrative embodiment, the proportion of the two compounds in the organo-substituted silane liquid is not critical and is dependent only upon having methyl groups present in sufficient quantity to furnish water repellency in the film. For example, a mixture containing from approximately 10 percent to 90 percent of methyltrichlorosilane and from approximately 90 percent to 10 percent of dimethyldichlorosilane yields good results, while satisfactory results are obtained when either dimethyldichlorosilane or methyltrichlorosilane is used alone as the organo-substituted silane liquid.

The thickness of the water repellent film which is formed on the silicon surface is very thin, being of the order of approximately 6 l() centimeters. For some applications, a film of less thickness may be allowable, or in some instances may be necessary. In such a case the thickness of the film may be decreased by diluting the silane liquid with as much as 90 percent of an organic solvent such as trichloroethylene.

After complete wetting of all surfaces of the silicon crystal has been achieved, the silicon crystal is removed from the water hydrolyzable organo-substituted silane liquid and baked to drive off all volatile materials, complete the polymerization of the silane, and strengthen the bond between the silicone polymer and the surface of the silicon crystal. In the presently preferred embodiment, for example, baking for approximately two hours at 100 C. or one hour at 300 C. accomplishes these results.

Although the present invention has been described with particular reference to the surface treatment of silicon crystals, germanium crystals and alloys of silicon and germanium as well as other semiconductor material may also be treated in accordance with this invention to obtain Water repellency and improved electrical characteristics.

Although dimethyldichlorosilane and methyltrichlorosilane have been described as the presently preferred silane liquid, other water hydrolyzable organo-substituted silanes, such as vinyltrichlorosilane, diethyldic'hlorosilane, ethyltrichlorosilane and triethoxymonohydrogensilane, may also be used. In all cases the organic group in the resulting silicone polymer will furnish the water repellency. By using these organo-substituted chlorosilanes, hydrochloric acid will again be split off in the presence of moisture and a silicone polymer film will cover the surfaces of the semiconductor body which is being treated. Ethoxy-substituted silanes may also be used and have been found to be particularly advantageous in the surface treatment ofgermanium crystal bodies. In this instance, by using triethoxymonohydrogensilane as the organo-substituted silane liquid in the method of surface treatment described hereinbefore, C H OI-I will be split off upon polymerization and the organic groups in the resulting silicone polymer film will furnish the water 'repellency.

In addition to obtaining a thermally stable water repellent film upon the semiconductor crystal body, the surface treatment described herein also yields improved electrical characteristics in semiconductor devices which are constructed by utilizing semiconductor crystal bodies which have been so treated, It has been found that when a semiconductor crystal body, such as silicon, having a P-N junction formed therein, is etched and exposed to ambient conditions, an effect known as the channel effect is encountered which produces poorly defined P-N junctions. Although the theory behind the channel effect in P-N junctions is not clearly understood, it has been found that .the surface treatment described herein eliminates such an effect and thereby maintains a clearly-defined P-N junction within the semiconductor body. Semiconductor devices utilizing semiconductor bodies treated in accordance with the present invention have improved saturation current characteristics and exhibit higher and harder peak inverse voltage characteristics. Further, the leakage across the surface of devices which are so treated is eliminated due to the high surface resistance.

Improvement of electrical characteristics through the organo-substituted silane film may be better seen through reference to the following table:

Where E is that voltage necessary to pass 10 micro amperes in the reverse'direction.

E is that voltage necessary to pass 2 micro amperes in the reverse direction.

1,, is that current flowing due to application of volts in the reverse direction.

From the foregoing table it can readily be seen that a semiconductor body including the organo-substituted film on its surface has greatly enhanced electrical characteristics. Although the above figures relate to a junction diode it has been found that semiconductor bodies containing the film which are to be utilized as transistors also have enhanced electrical characteristics.

Although the silane film is applied to the semiconductor body by immersion of the body of the silane liquid in this illustrative embodiment, it may also be applied by entraining the silane in vapor form and causing the entrained vapor to be passed over the surfaces of the semiconductor body.

Thus, the present invention provides semiconductor crystal bodies containing a film of organo-substituted silane on at least one surface thereof which have greatly improved electrical characteristics.

What is claimed is:

1. In a semiconductor translating device, the combination comprising: a semiconductor body of one conductivity type; a region within said body of the opposite conductivity type and separated from said body by a rectifying junction; and a film upon at least a portion of one surface of said body, said film comprising a hydrolyzed organo-substituted silane.

2. In a'semiconductor translating device, the combination comprising: a semiconductor body of one conductivity type; a region within said body of the opposite conductivity type and separated from said body by a rectifying junction; and a film covering at least a portion of said body including said junction, said film comprising a hydrolyzed organo-substituted silane.

3. In a'semiconductor translating device, the combination comprising: a semiconductor body of one conductivity type; a region within said body of the opposite conductivity type and separated from said body by a rectifying junction; and a film upon at least a portion of one surface of said body, said film comprising a hydrolyzed organo-substituted chlorosilane.

4. In a semiconductor translating device, the combination comprising: a semiconductor body of one conductivity type; a region within said body of the opposite conductivity type and separated from said body by a rectifying junction; and a film upon at least a portion of one surface of said body, said film comprising a hydrolyzed polymerized dimethyldichlorosilane methyltrichlorosilane solution.

5. In a semiconductor translating device, the combination comprising: a semiconductor body of one conductivity type; a region within said body of the opposite conductivity type and separated from said body by a rectifying junction; and a film upon at least a portion of one surface of said body, said film comprising a heat treated polymerized dimethyldichlorosilane methyltrichlorosilane solution.

No references cited. 

1. IN A SEMICONDUCTOR TRANSLATING DEVICE, THE COMBINATION COMPRISING: A SEMICONDUCTOR BODY OF ONE CONDUCTIVITY TYPE; A REGION WITHIN SAID BODY OF THE OPPOSITE CONDUCTIVITY TYPE AND SEPARTED FROM SAID BODY BY A RECTIFYING JUNCTION; AND A FILM UPON AT LEAST OF ONE SURFACE OF SAID BODY, SAID FILM COMPRISING A HYDROLYZED ORGANO-SUBSTITUTED SILANE. 